Electric coupling system



June 10,1930. c. E. TRUBE 1,763,330

I ELECTRIC COUPLING SYSTEI Filed July 14. 1928 2 Sheets-Sheet 1 INVENTORATTORNEYS June 10,1930. c. E. TRUBE 1,763,330

' ELECTRIC COUPLING SYSTEI Filed July 14, 1928 2 Sheets-Sheet 2 INVENTOR(br/ E @056 0M, 777 ZWQ ATTORNEYS the stability and the selectivity.

Patented June 10, 1930 PATENT OFFICE .UNITED STATES CARL E. TRUBE, OFORANGE, NEW JERSEY, ASSIGNOR TO HAZELTINE CORPORATION, OF JERSEYCITY,'NEW JERSEY, A CORPORATION OF DELAWARE ELECTRIC courtme SYSTEMApplication filed July 14, 1928, Serial No. 292739, and in Great BritainJuly 2, 1926.

This invention relates to electric coupling systems especially adaptedfor use with radio-frequency vacuum tube amplifiers, and moreparticularly to circuit arrangements which operate most effectively overa relatively wide range in frequency. Such systems are discussed moregenerally in my copending application for U. S..Letters Patent SerialNo. 120,045, filed July 2, 1 926, of

which the present application is a continuation in part; Figs. 2 and 5of this present application being the same as Figs. 6 and 8' of theprior application, and present Fig. 1 being the same as prior Fig. 2except forthe substitution of a radio-frequency selfinductance for thechoke coil described in the prior specification.

- The mentioned prior application discloses various forms of couplingsystems so arranged as to give a voltage ratio that varies automaticallywith the frequency. The present application is confined to'such couplingsystems in which the variation in voltage ratio is accomplished in aprimary circuit which includes a fixed self-inductance effectively inparallel with one or more fixedcircuit is resonant at a frequency lower(but preferably not greatly lower) than the lowest frequency within therange of the CO2- pling system. I

The voltage ratio' in an inter-tube coupling system affects the degreeof amplification, At each frequency there is a particular ratioo'f-output voltage to input voltage that will give the best compromisedesign. This ratio varies rapidly with the frequency, being relativelyhigh at high frequencies. This invention provides means for securingsubstan- .tially the most desirable ratio at all freby which directvoltage is removed from the fixed capacities.

Fig. 5 illustrates a form of this invention in which a self-inductancecoil (L of the preceding figures, is electromagnetically coupled to thetransformer.

Fig. 6 illustrates a modification of Fig. 5.

Fig. 7 illustrates another modification of Fig. 5.

The coupling system of Fig. 1 is tuned by a variable condenser Cconnected across the primary coil L and the secondary coil L in series.Ordinarily L should have much fewer turns than L so that there is a hi hstep-up voltage ratio between them. e path through the primary coil Lfrom the plate of the vacuum tube to the filament includes also thefixed capacity G which is ex ternal to the main resonant circuit C L LIn parallel with this path is a second path including theself-inductance L the value of which is so chosen that its circuit(completed through C and L is resonant at a frequency lower, but notgreatly lower, than the lowest frequency within the range of condenser CThe circuit of L being completed through C and L is thereby coupled tothe main resonant circuit C L L so that part of the current of C and Lwill flow through C and L this part of the currentrbeing limited mostlyby the reactance of L Since the reactance of L varies directly with thefrequency while that of C varies inversely with the frequency, thevoltage across C will rapidly fall off relative to the voltage across Las the frequency rises. For a given output voltage E the voltage acrossL is very. nearly independent of frequency, due to the electromagneticcoupling between L and L Hence the voltage E between the plate and thefilament, being the sum of the voltages of indicated, being such voltageacross 0 alone. This is the condition which is desired and which resultsin the maintenance of a high degree of stability over the frequencyrange, together with high sensitivity and selectivity.

The operation of the circuit of Fig. 2 is essentially the same as thatof Fig. 1, but here the circuit of L parallel paths, that through C andthat through C and L As before, this circuit L C C L should be maderesonant at a frequency lower, but not greatly lower, than the lowestfrequency within the range of C As described in the above-mentionedprior application, the voltage ratio E /E in Fig. 2 is obtained from adouble step-up, first between C and the combination of G and C and thenbetween L and L and, with the resonant condition just given, thisvoltage ratio will automatically vary with the frequency in the samemanner as in Fig. 1.

Fig. 3 differs from Fig. 2 only in that the primary coil L is notincluded in the main resonant circuit. Coil L should here have the samenumber of turns as in Fig. 2; while the secondary coil L should have thesame number of turns as L Fig. 2. The remaining relations are the sameas in Fig. 2.

In Fig. 3 the direct voltage of the plate battery B exists acrosscondensers C and G which is disadvantageous in that these condensers maybreak down. This difficulty is obviated in Fig. 4 by connecting thelower terminals of condenser C and coil L to the lower terminal of coilL instead of to the common lead at the bottom of the diagram. Thisconnection does not aifect the alternating-current relations of Fig. 4,which are identical with those of Fig. 3.

In Figs. 1 to 4, the coupling between the circuit of coil L and the mainresonant circuit C L L or C L. is effected through the coil L togetherwith its electromagnetic couplin to coil L However, it is possible tocouple t e circuit of L directly with the main resonant circuit bycoupling L electromagnetically to L One method of accomplishing this isillustrated in Fig. 5, whose elements correspond to those ofFig. 1 andare denoted by like reference characters. Fig. 5 also differs from Fig.1 in isolating the primary system C L L from the secondary system 0 L ina way similar to that shown in Fig. 4. As in the circuit arrangement ofthe preceding figures, L of Fig. 5 should ordinarily have much fewerturns than L and the circuit of L including L and 0,, should be maderesonant at a frequency somewhat lower than the lowest frequency withinthe range of C The proper relative polarities of coils L and L; are asthat the terminal .of E connected to one side of C is of oppositepolarity to the terminal of L connected to is completed through two- 1The operation of Fi and L combined, in

the other side of C the word polarity here referring to the polarity ofthe respective terminals when the coils are on open circuit. As before,there is in Fig. 5 a path from the plate to the filament whichincludesin series the fixed condenser C and the primary coil L With the relativepolarities just stated,

the voltages across L and C are additive, due to the fact that atfrequencies higher than the resonant frequency of the circuit C L theterminal voltage of L is reversed with respect to the voltage induced inL; by the current in L 5 are thus similar to those of the precedingfigures, although the mathematical equations diiier slightly in form.

As the self-inductance of coil L in Fig. 5 should be made much greaterthan that of coil L (which ordinarily has but few turns) it will makevery little difference whether the lower terminal of L is connectedtothe upper terminal of L as in Fig. 5, or to the lower terminal of L asin Fig. 6, so long as the correct polarity relations are maintained. 6is qualitatively identical with that of ig. 5 and can be madeqllllantitatively identical by slight chan es in t e values of thecircuit constants. s before, the circuit of coil L which is completedthrough condenser C is resonant at a freqiliency lower, but not greatlylower, than t e lowest frequency within the range of 0 Since the circuitof L does not now include the coil L it is necessary to couple it to thesecondary circuit C- L by electromagnetic coupling between the coils Land L In cases where the natural capacities to ground of coil L andcondenser C, are objectionable, it may be desirable to interchange theorder of this circuit and L in Fig. 6, giving the arrangement of Fig. 7.Except for the minor effects of these capacities to ground, theoperation of Fig. 7 is identical with the operation of Fig. 6 if therelations of the parts are otherwise the same as in Fig. 6.

The use of electromagnetic coupling between coils L andL in Figs. 5, 6and 7 results in a more gradual variation with frequency of the voltageratioE /E and there'- fore of the performance, i. e., of the stability,sensitivity and selectivity. The coeflicient of coupling between L and Lshould be neither very close nor very loose; a coupling of the order of30% being satisfactory. It is to be understood that as manyradio-froquency stages as required, similar to those .herein described,may be connected in cascade-the output terminals of one stage connectedto the input terminals of the next-in order to obtain the advantagesofthis invention in a multi-stage amplifier.

I claim: 1. In a radio amplifier stage including a vacuum tube having afilament, a grid'and a The voltage relations of Fig.

plate, an electric coupling system which coniprises a main resonantcircuit including as elements a secondary coil and a condenser, at leastone of which is adjustable to tune the coupling system over a range infrequency,

a second circuit coupled to said resonant circuit, including a fixedself-inductance effectively in parallel with fixed capacity external tosaid main resonant circuit, whereby said secondcircuit is resonant at afrequency lower than the lowest frequency within said range, a primarycoil electromagnetically.

coupled to said secondary coil, and a path through said coupling systembetween said plate and said filament includingin series said primarycoil and at least aportion of said fixed capacity, whereby in theoperation of the amplifier stage there is developed in said mainresonant circuit a resonant voltage whose ratio to the voltage betweenthe plate and the filament automaticall rises when said adjustableelement is adjusted for higher frequencies.

2. In a radio amplifier stage including a vacuum tube having a filament,a grid and a plate, an electric coupling system which comprises a mainresonant circuit including a xed secondary coiland an adjustablecondenser adapted to tune the coupling system over a rangein frequency,a circuit coupled to said resonant circuit, including a fixedself-inductance effectively in parallel with fixed capacity external tosaid main resonant circuit, whereby said second circuit is resonant at afrequency lower, but not greatly lower, than the lowest frequency withinsaid range, a primary co-il electromagnetically coupled to saidsecondary coil and having few turns relatively thereto, and a paththrough said coupling system between said'plate and said filamentincluding in series said rimary coil and at least a portion of said xedcapacity, whereby in the operation of the amplifier stage there isvdeveloped in said main resonant circuit a resonant voltage highcompared with the voltage between the plate and the filament, andwhereby the ratio of said resonant voltage to the voltage between theplate and the filament automatically rises rapidly when said adjustablecondenser is adjusted for higher frequencies.

3. In a radio amplifier stage including a vacuum tube having a filament,a grid'and a plate, an electric coupling system which comprises a mainresonant circuit including a fixed secondary coil and an adjustablecondenser adapted to tune the coupling system over a range in frequency,a fixed coil electromagnetically coupled to said secondary coil, acircuit including said fixed coll andja fixed capacity effectivelyinparallel, said N fixed capacity being external to said main resonantcircuit, a primary coil electromagnetically coupled to said secondaryc011, and a path through said coupling system between said plate andsaid filament including in series said fixed capacity and said primarycoil, whereby in the operation of the amplifier stage there is developedin said resonant circuit a resonant voltage whose ratio to the voltagebetween the plate and the-filament automatically rises when saidadjustable con denser is adjusted for higher frequencies.

4. In a radio amplifier stage including a vacuum tube having a filament,a grid and'a plate, an electric coupling system which comprises a' mainresonant circuit including a fixed secondary coil and an adjustablecondenser adapted to tune the coupling system over a range in frequency,a fixed coil electromagnetically coupled to said secondary coil, acircuit including said fixed coil and a fixed capacity effectively inparallel, and having a resonant frequency lower than the lowestfrequency within said range, said fixed-capacity being external to saidmain resonant circuit, a primary coil electromagnetically coupled tosaid secondary coil, and a path through said coupling system betweensaid plate and said filament including in series said fixed capacity andsaid primary coil, whereby in the operation of the amplifier stage thereis developed in said resonant circuit a resonant voltage whose ratio tothe voltage between the plate and the filament automatically rises whensaid adjustable condenser is adjusted for higher frequencies.

fixed secondary coil and an adjustable condenser adapted to tune thecoupling system over a range in frequency, a fixed coilelectromagnetically coupled to said secondary coil, a circuit includingsaid fixed coil and a fixed capacity effectively in parallel, said fixedca pacity being external to said main resonant circuit, a primary coilelectromagnetically coupled to said secondary coil, and a path 'throughsaid coupling system-between said plate and said filament including inseries said fixed capacity and primary 0011, said primary coil havingsuch polarity relative to said fixed coil that the terminal of the stagethere is developed in said resonant circuit a resonant voltage whoseratio to the voltage between the plate and the filament automaticallyrises when said adjustable condenser is adjusted for higher frequencies.

6. In-a radio amplifier stage including a vacuum tube having a filament,a grid and a plate, an electric coupling system which comprises a mainresonant circuit including a fixed secondary coil and an adjustablecondenser adapted to tune the coupling system over a range in frequency,a fixed coil electromagnetically coupled to said secondary coil, acircuit including said fixed coil and a fixed capacity e liectivelyinparallel and having a resonant frequency lower than the lowest frequencywithin said range, said fixed capacity being external to said mainresonant circuit, a primary coil electromagnetically coupled to saidsecondary coil, and a path through said coupling system between saidplate and said filament including in series said fixed capacity and saidprimary coil, said primary coil having such polarity relative to saidfixed coil that the voltages across said primary coil and said fixedcapacity are additive, whereby in the operation of the amplifier stagethere is developed in said resonant circuit a resonant voltage whoseratio to the voltage between the plate and the filament automaticallyrises when said adjustable condenser is adjusted for higher frequencies.

7. In a radio amplifier stage including a vacuum tube having a filament,a grid and a plate, an electric coupling system which comprises a mainresonant circuit including as elements a coil and a condenser, at leastone of which is adjustable to tune the coupling system over a range infrequency, a second circuit coupled to said resonant circuit including afixed self-inductance efiectively in parallel with fixed capacityexternal to said main resonant circuit, whereby said second circuit isresonant at a frequency lower, but not greatly lower, than the lowestfrequency within said range, a circuit element having a substantiallyfixed voltage ratio relative to said main resonant circuit, and a paththrough said coupling system between said plate and said filamentincluding in series said circuit element and at least a pbrtion of saidfixed capacity, whereby in the operation of the amplifier stage there isdeveloped in said main resonant circuit a resonant voltage whose ratioto the voltage between the plate and the filament automatically riseswhen said adjustable element is adjusted for higher frequencies.

8. In a radio amplifier stage including a vacuum tube having a filament,a grid and a plate, an electric coupling system which comprises a mainresonant circuit including as elements a coil and a condenser, at leastone 'of which is adjustable to tune the coupling system over a range infrequency, a second circuit coupled to said resonant circuit including afixed self-inductance effectively in parallel with fixed capacityexternal to said main resonant circuit, whereby said second circuit isresonant at a frequency lower, but not greatly lower, than the lowestfrequency within said range,-said fixed self-inductance beingelectromagnetically coupled to said resonant circuit, a circuit elementhaving a substantially fixed voltage ratio relative to said mainresonant circuit, and a path through said coupling system between saidplate and said filament including in series said circuit element and atleast a portion of said fixed capacity, whereby in the o ration of theamplifier stage there is deve oped in said main resonant circuit aresonant voltage whose ratio to the voltage between the plate and thefilament automatically rises when said adjustable elements is adjustedfor higher frequencies.

9. A radio-frequency coupling system having input terminals andcomprising a main resonant circuitincluding as elements a coil and acondenser, at least one of which is adjustable to tune the couplingsystem over a range in frequency, a second circuit coupled to saidresonant circuit including a fixed self-inductance effectively inparallel with fixed capacity external to said main resonant circuit,whereby said second circuit is resonant at a frequency lower, but notgreatly lower, than the lowest frequency within said range, a circuitelement having a substantially fixed voltage ratio relative to said mainresonant circuit, and a path through said coupling system between saidinput terminals and including in series said circuit element and atleast a portion ofsaid fixed capacity, whereby there is developed insaid main resonant circuit a resonant voltage whose ratio to the voltagebetween said terminals automatically rises when said adjustable elementis ad usted for higher frequencies.

In testimony whereof I afiix my si CARL E. TR BE.

ature.

